Flash Memory-Interface

ABSTRACT

Flash-type memory access and control is facilitated (e.g., as random-access memory). According to an example embodiment, an interface communicates with and controls a flash memory circuit over a peripheral interface bus. The interface uses a FIFO buffer coupled to receive data from and store data for the flash memory circuit and to provide access to the stored data. An interface controller communicates with the flash memory circuit via the peripheral interface bus to initialize the flash memory circuit and to access data thereto, in response to requests from a processor. In some applications, the flash memory circuit is initialized by sending commands to it. The interface may be placed into a read-only mode in which data in the flash memory is accessed as part of main (computer) processor memory, using the FIFO to buffer data from the flash.

FIELD OF THE INVENTION

The present invention relates generally to memory devices, and morespecifically to a memory device having a flash-type interface.

BACKGROUND

Flash memory is a non-volatile type of erasable and rewritable memory,which has experienced a good deal of success due to its relativelyinexpensive nature. In many instances, flash memory is erased andprogrammed in large blocks. Implementations of flash memory haveincluded external memory such as memory cards and flash drives, as wellas internal memory, such as solid state storage for hand-held or mobiledevices.

Flash memory devices are operated in a variety of manners. In manyapplications, a command mode is used for operations such as erasing andprogramming, and a memory mode is used for reading data from the flash.In some applications, flash devices use a half-duplex command-drivenserial peripheral interface (SPI) protocol for devicesetup/initialization, and then move to a half-duplex command-driven4-bit protocol for normal operation. However, different flash devicesaccept and/or require different commands and command formats. In manycases, flash devices are not compatible with certain types of otherdevices or processors. In addition, many forms of flash memory are notamenable to rapid access as required or desirable in certainapplications.

These and other issues continue to present challenges to the utilizationof serial flash memory.

SUMMARY

The present invention is exemplified in a number of implementations andapplications, some of which are summarized below.

Consistent with an example embodiment of the present invention, aperipheral interface circuit interfaces between a computer processor anda flash memory circuit that is communicatively connected to theinterface by a peripheral interface bus, in order to store and provideaccess to data that is stored on the flash memory circuit, which ismapped as a portion of main processor memory. The interface circuitincludes a first-in, first-out (FIFO) buffer and an interfacecontroller. The FIFO buffer is coupled to receive and store data fromthe flash memory circuit and to provide data buffering. The interfacecontroller is configured to communicate with the flash memory circuitvia the peripheral interface bus, initialize the flash memory circuitfor providing data to the FIFO buffer as a portion of the main memory,and provide data from the flash in response to requests for access toaddresses mapped as a portion of main memory. In response to a requestfor the data mapped as a portion of main memory and having at least aportion thereof stored in the FIFO buffer, the controller controls theFIFO buffer to provide access to the stored data and (if necessary)controls the flash memory circuit to provide the balance of therequested data from subsequent addresses in the flash memory circuit tothe FIFO buffer. In response to a request for the data mapped as aportion of main memory and not stored in the FIFO buffer, the controllercontrols the flash memory circuit to provide the requested data to theFIFO buffer. In either case, the controller reads the contents ofsubsequent addresses and stores them in the FIFO buffer.

Another example embodiment is directed to a flash memory-interfacesystem for storing and providing access to data mapped as a portion ofmain memory of a computer. The system includes a peripheral interfacebus, a flash memory circuit, a FIFO buffer and an interface circuit. Theflash memory circuit is configured to connect to and communicate via theperipheral interface bus and to store the data mapped as a portion ofmain memory. The FIFO buffer and interface circuit generally operate asdescribed above, in connection with the peripheral interface circuit.

Other example embodiments are directed to methods of controllinginterface and/or memory circuits in accordance with one or moreapproaches as discussed herein.

The above summary is not intended to describe each embodiment or everyimplementation of the present disclosure. The figures and detaileddescription that follow more particularly exemplify various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 shows a plot of signals for operating a flash memory circuit inSPI and quad modes, in accordance with one or more example embodimentsof the present invention;

FIG. 2 shows a plot of signals for interfacing with and accessing aflash memory circuit in serial peripheral interface (SPI) and quadmodes, in accordance with other example embodiments;

FIG. 3 shows a block diagram of a peripheral flash interface (SPIFI)module, according to another example embodiment;

FIG. 4 shows a block diagram utilizing a SPIFI module, according toanother example embodiment of the present invention;

FIG. 5 shows a data-flow diagram for initial setup of a processor-flashmemory module, according to another example embodiment of the presentinvention;

FIGS. 6A and 6B show a flash memory interface system operating in memorymode, according to another example embodiment of the present invention;and

FIG. 7 shows a FIFO block diagram for implementation with a flash memorycircuit, according to another example embodiment of the presentinvention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe scope of the invention including aspects defined in the claims.

DETAILED DESCRIPTION

The present invention is believed to be applicable to a variety ofdifferent types of processes, devices and arrangements for use withcomputer memory applications. While the present invention is notnecessarily so limited, various aspects of the invention may beappreciated through a discussion of examples using this context.

In connection with various example embodiments, a flash memory interfaceis configured to interact between a computer processor (CPU), optionallya memory controller thereof; and a flash memory circuit, for writingdata to and reading data from the flash memory circuit. The flash memoryinterface includes a communication port for communicating with the flashmemory circuit, a first-in, first-out (FIFO) type buffer to store datatransferred to and from the flash memory circuit, and a controllerconfigured to control the writing of data to and the reading of datafrom the flash memory circuit, in response to the CPU or memorycontroller.

The flash memory interface is configured to interact with one or moretypes of flash memory circuits, and provides a consistent interfaceacross all such flash memory circuits for access by one or more of avariety of types of CPUs and/or peripheral devices. The flash memoryinterface is responsive to read and write commands from the CPU byinteracting with a flash memory circuit via the communications port.Communications are in accordance with serial and 4-bit bidirectionalprotocols. For instance, a half-duplex command-driven serial peripheralinterface (SPI) protocol can be used for device setup/initialization,followed by the use of a half-duplex command-driven 4-bit protocol fornormal operation. Operations with the bus master may include byte,halfword, and word accesses.

The flash memory circuits respond to software-driven commands (e.g.,from a computer processor accessing the flash) and/or commandsautomatically sent by the flash memory interface when the flash regionof the memory map is read. In some instances, commands are divided intofields including opcode, address, intermediate data, and data. Theaddress, intermediate data, and data fields are optional depending onthe opcode. In some implementations, the opcode is implied in readcommands for higher performance. Data fields can be divided into inputand output data fields depending on the opcode.

In one particular implementation, the flash memory interface operates ina master mode under half-duplex communications with a flash memoryslave. During initial setup, the flash memory interface is initializedsuch that thereafter, the flash memory effectively inhabits a portion ofmemory space of the CPU (e.g., as a portion of memory space of aprocessor such as a RISC-type (e.g., from ARM Holdings of San Jose,Calif.), acting in a manner similar to an External Memory Interface orSDRAM controller).

When erasing or programming is needed, the flash memory interfaceoperates in a peripheral mode, in which registers in the interface arewritten (e.g., via software or firmware) with a proper sequence ofvalues to accomplish the programming. During erasure or programming, theflash address block is not read.

The flash memory interface provides a flash address block for the flashmemory circuit (or circuits) coupled thereto, for reading by the CPU asinhabiting a portion of its memory space (e.g., as a portion of ARMmemory space as discussed above). In some implementations, the CPUaccesses the flash address block via sequential or random loadinstructions, and either processes the data directly or copies the datato random-access memory (RAM). In other implementations, a DMAcontroller sequentially reads data from the flash memory circuit(s)independently of the CPU, and copies the data to RAM or outputs the datato a peripheral. In still other implementations, code is executeddirectly from the flash memory circuit, such as for operations that donot require high speed (e.g., for non-critical code and device or systemsetup).

The flash memory interface includes a first in, first out (FIFO) buffer.After a request for data on a coupled flash memory circuit is completed,the flash memory interface speculatively reads data following theaddress requested by the bus master and stores it in the FIFO. Thisapproach accommodates expected sequential access to the flash memorycircuit. If the CPU requests an address of data that is already in theFIFO, the data is provided from the FIFO, which would then be emptied upthrough and including the data provided and refilled accordingly.

If the CPU requests an address other than any of those in the FIFO, theflash memory interface flushes the FIFO, de-asserts the chip select toterminate the read command, and issues a new read command with the newaddress. If the FIFO becomes full, the flash memory interfacediscontinues clocking the flash memory. A timeout for “FIFO full” statede-asserts the chip select to the flash, which places the flash memoryin low-power mode.

A generalized manner in which to indicate the format of each command isused by the flash memory interface for both direct operation undersoftware control, and for control information for initiating readoperations in memory mode. This generalized approach providesindependence from any particular command set used by a flash memory,including whether each part of a command is transferred in serial orquad I/O fashion.

Data transfer is effected in accordance with the particular flash devicebeing used, including capabilities of the device such as the format ofcommands that the device supports. In some embodiments, initial setupincludes a command having a format that is provided by all flashdevices, with which the flash interface can be used. After the flashinterface sends this command, the flash memory responds with a value(e.g., a 3-byte value) that indicates the flash device's identity andthus its capabilities. The ID value is used (e.g., viasoftware/firmware) to guide the commands issued by the interface.

Table 1 shows pin descriptions and signals sent between a serialperipheral interface flash interface (SPIFI) and a flash memory, for usein connection with one or more example embodiments of the presentinvention. In some implementations, a pin multiplexing logic circuitalternates these pin functions with one or more other optionalfunctions.

TABLE 1 Device Pin Descriptions Pin name Type Pin Description SCK OutSerial clock for the flash memory, switched only during active bits onthe MOSI/IO0, MISO/IO1, and IO3:2 lines. CSn Out Chip Select for theflash memory, driven low while a command is in progress, and highbetween commands. In the case of one serial slave, this signal can beconnected directly to the device. If more than one serial slave isconnected, off-chip hardware and software- implemented functions can usegeneral-purpose I/O signals in combination with this signal to generatethe chip selects for the various slaves. MOSI/IO0 I/O This is an outputexcept in quad input data fields. After a quad input data field in asingle-master application, it becomes an output again one serial clockperiod after CSn goes high. MISO/IO1 I/O This is an output in quadopcode, address, intermediate, and output data fields, and an input ininput data fields. After an input data field in a single-masterapplication, it becomes an output again one serial clock period afterCSn goes high. IO3:2 I/O These are outputs in quad opcode, address,intermediate, and output data fields, and inputs in quad input datafields. After a quad input data field in a single-master application, itbecomes an output again one serial clock period after CSn goes high. Ifthe flash memory does not have quad capability, these pins can beassigned to GPIO or other functions. GRANTn In Optional input used bydevices that are designed to permit multi-master operation. See theGRANTn description in Table 7 for more information.

FIGS. 1 and 2 show interface signals for operating a flash memorycircuit, as may be implemented in accordance with one or more exampleembodiments, including those shown in and described in connection withthe remaining figures. FIG. 1 shows the transmission of a commandconsisting of an opcode byte 0x06, respectively sent inserial-peripheral interface (SPI) mode 100 and quad mode 110, inaccordance with one or more example embodiments. Bytes are sent with themost significant bit first in a serial-peripheral interface SPI mode,and the most significant bits first in quad mode.

In the above examples, and as may be applied to the various embodimentsdescribed herein, the term “SPI” as well as “SPIFI” may refer to devicesoperating using one or more of serial communications, dual-modecommunications, quad mode communications or others. Accordingly, theapplication of a “serial peripheral interface” as relevant to theabbreviation “SPI” is not limited to serial communications, as might beconstrued from common use of the term.

FIG. 2 shows interface signals used for two commands that read data froma flash device, in accordance with other example embodiments. In SPImode, 1 byte is read from the slave. In quad mode, 3 bytes are read fromthe slave with the opcode and input data fields both in quad mode. Inquad mode, the IO3:0 lines are driven by the interface in opcode,address, intermediate and output data fields, and driven by the flashmemory in input data fields. In address fields, the more significantbytes are sent first.

FIG. 3 shows a peripheral flash interface (SPIFI) arrangement 300,according to another example embodiment. The SPIFI 300 interfaces with aflash memory device in command mode and memory mode (e.g., readingdata), as respectively controlled by command mode and memory mode statemachines 310 and 320, which operate in accordance with registers 312 anda memory slave region 322 of the device.

The SPIFI 300 also includes a flash bus state machine 330, a FIFO memory340 and shift register(s) 350. The flash bus state machine passes clockand chip select signals to a flash memory circuit, and further passessignals for controlling the FIFO memory 340 and shift registers 350 topass data to and from the flash memory circuit.

The SPIFI 300 can be operated in accordance with a variety of differentsystems, flash memory circuits, and operational commands, some of whichare described above, in the following discussion, and/or in connectionwith the remaining figures. In this context, FIG. 4 shows a blockdiagram of a simulation environment with a SPIFI device represented byblock 402, and a serial flash device represented by block 404, havingrespective input/output pins as can be operated, for example, inaccordance with the above discussion and tables herein. Additionalblocks provide various clock signals and bus functions. The approachshown in FIG. 4 may, for example, be used in connection with a processorcircuit 406 such as employed in a computer system and optionally anotherdevice such as a display device 408. Where used with a display device,the SPIFI 300 may, for example, interface with a flash memory circuitfor storing image data.

According to another example embodiment of the present invention, anexternal serial flash device is initialized during device setup. In oneimplementation, the serial flash device is initialized using commandsthat place the device in a high-performance mode. After theinitialization command sequence is complete, a command to be issued, inresponse to a read from the serial flash region of the memory, iswritten to a memory command register. In some implementations, attemptsto read the flash region before the memory command register is writtenare aborted.

After the memory command register is written, read access to the serialflash contents is enabled. Such access may include load commands, directmemory access (DMA, such as by programming a DMA channel to read serialflash data and write it to RAM or a peripheral device), and/orinstruction execution from the serial flash address region. In oneimplementation, code is executed directly from the serial flash for codesequences that do not have stringent performance requirements.

Referring again to FIG. 3 by way of example and in connection with anexample embodiment, a FIFO buffer 340 provides buffering between theflash memory and other components of the system. The SPIFI 300 maintainsa register containing the number of bytes in the FIFO. The SPIFI 300provides access, such as to a processor executing software or a DMAcontroller, to read bytes, halfwords, or words from the flash region.When a read from the flash region presents an address for which thecorresponding data is not available in the FIFO 340, the SPIFI 300terminates any command in progress, discards any data in the FIFO, andthen issues the command in the memory command register, using the numberof least significant bits of the access address indicated by the AMSBfield (see, e.g., Table 2) of the control register. Any more significantaddress bits in the number of address bytes selected by the memorycommand register are sent as zeroes. Reads from the flash region aredelayed if necessary, until (all of) the requested byte(s) is (are)available in the FIFO 340. At that time the SPIFI 300 removes byte(s)from the FIFO 340 through the last one requested. In the event thatthere are bytes in the FIFO 340 at lower addresses than the requestedaddress, these bytes are discarded.

After returning data from the FIFO 340, the SPIFI 300 readshigher-addressed data from the flash device (coupled at 312) and storeit in the FIFO. In the event that the FIFO 340 becomes full, the SPIFI300 stops clocking the serial flash device, so that no further data isread from the flash until the processor (or DMA controller) reads fromthe flash region to make room in the FIFO.

In one implementation, the SPIFI 300 drives a chip select (CS) pin(discussed above) via flash bus state machine 330 high to terminate thecommand and place the serial flash in low-power mode, when the FIFO 340remains full with no further reading after a read from the flash region,or when the data register is not read after a command is written to thecommand register that specifies an input data field, for the length oftime specified by the TO field (see Table 2) of the control register.Termination of the command will request an interrupt if the INTEN bit(see Table 2) in the command register is set.

The effect of such termination for reading from the flash region is thatthe SPIFI 300 re-issues the command in the memory command register whena new access to the region occurs. However, if the data register is reador written when an appropriate command is not in progress, the SPIFI 300issues an abort exception.

According to another example embodiment of the present invention, theSPIFI 300 is used to program or erase a serial flash device in a commandmode as follows. An appropriate sequence of commands is written tocommand, address, and data registers (e.g., at 312), via registeraddresses outside access the flash region of the address map. When thissequence is completed and an actual erase or program operation is underway in the serial flash device, a Read Status command is issued with apollRS bit (see Table 3) set in the command register. The SPIFI 300interrupts the processor when the erase or write operation (and thus theRead Status command) completes if INTEN in the control register is 1, orotherwise permits continual or periodical reading of the status registeruntil it indicates that the Read Status command is complete. Aftercompletion of erasing or programming, further programming or erasing canbe carried out, or the device may return to memory mode operation, inwhich data stored in the flash can be read.

The registers 312 may include one or more of a variety of registers(e.g., a control register, command register, address register,intermediate data register, address and intermediate data register, dataregister, memory command register, and status register), depending uponthe application, some of which are discussed above. In oneimplementation, the registers 312 include control, command, and dataregisters as follows. The control register controls the overalloperation of the SPIFI 300. The control register is written before anycommands are initiated, and includes the fields shown in Table 2.

TABLE 2 Control Register Bit(s) Name Description 26:24 CSHI This fieldcontrols the minimum CSn high time, expressed as a number of serialclock periods minus one. 23 MODE3 If this bit is 0, the SPIFI drives SCKlow after the rising edge at which the last bit of each command iscaptured, and keeps it low while CSn is high. If this bit is 1, theSPIFI keeps SCK high after the rising edge for the last bit of eachcommand and while CSn is high, and drives it low after it drives CSnlow. 22 INTEN If this bit is 1 when a command ends, the SPIFI willassert its interrupt request output. See INTRQ in Table 7 for furtherdetails. 21 CLRID If this bit is 1, the Intermediate Data register iscleared by Reset and by a write to the Command register, so that futurecommands containing intermediate data will have zeroes in such fieldsunless the register is re-written. If this bit is 0, the register iscleared automatically only by Reset. 20:5  TO This field contains thenumber of HCLK periods without the processor or a DMA channel taking orproviding data during a command, which will cause the SPIFI hardware toterminate the command by driving the CS pin high and negating the CMDbit in the Status register. This allows the flash memory to enter alower-power state. The timeout applies after the FIFO has been filledduring a read command from the serial flash, or the FIFO has beenemptied during a write command to the serial flash. The command in thememory command register is issued again if the processor or DMA channelsubsequently comes to read data from the flash region again. The SPIFIhardware responds with a Data Abort if the processor or a DMA channelreads or writes the Data register when an appropriate command is not inprogress. 4:0 AMSB This field identifies the most significant addressbit of a complete internal flash address, and can be set to match the(total) size of the attached device(s). When sending an address in acommand, the SPIFI will send zeroes in bits above this number. 23₁₀ =0x17 for 128 Mbits (16 Mbytes) 22₁₀ = 0x16 for 64 Mbits (8 Mbytes) 21₁₀= 0x15 for 32 Mbits (4 Mbytes) 20₁₀ = 0x14 for 16 Mbits (2 Mbytes) 19₁₀= 0x13 for 8 Mbits (1 Mbytes)

The command register includes the fields shown in Table 3. In someimplementations, the command register can only be written as a word, butbytes, halfwords, and words can be read from the command register. Whenthe command register is written, any previous command in progress withthe serial flash device is terminated, memory mode is cleared, data inthe FIFO 340 is discarded, and the new command is transmitted. In someimplementations, for a command that contains an address and/orintermediate data, Address and/or Intermediate Data Register(s) arewritten before the command register is written. In the event that thecommand contains output data, the output data is written to the dataregister after writing to the command register, and if the commandcontains input data, the input data is read from the data register afterwriting to the command register.

TABLE 3 Command Register Bit(s) Name Description 31:24 opcode The opcodeof the command (not used for some frameForm values). 23:21 frameFormThis field controls the opcode and address fields: 000 used in all-zerovalue to terminate a command or memory mode 001 opcode only 010 opcode,LS byte of address 011 opcode, 2 LS bytes of address (MS byte first) 100opcode, 3 LS bytes of address (MS byte first) 101 opcode, 4 bytes ofaddress (MS byte first) 110 no opcode, 3 LS bytes of address (MS bytefirst) 111 no opcode, 4 bytes of address (MS byte first) 20:19 P/S Thisfield controls how the fields of the command are sent: 00 all fields ofthe command are serial 01 data field is quad on IO3:0. other fields areserial on IO0 10 opcode field is serial on IO0, other fields are quad onIO3:0 11 all fields of the command are in quad format 18:16 intLen Thisfield controls how many “intermediate” bytes precede the data. (Eachsuch byte may require 8 or 2 SCK cycles, depending on whether theintermediate field is in serial or 4-bit format.) Intermediate bytes areoutput by the SPIFI, and include post-address control information,“dummy” and delay bytes. See the description of the Intermediate Dataregister for the contents of such bytes. 15 DOUT If the dataLen field isnonzero, this bit controls the direction of the data: 0 = input fromserial flash 1 = output to serial flash 14 pollRS This bit is written as1 only with an opcode that a) contains an input data field, and b)causes the serial flash device to return byte status repetitively (e.g.,a Read Status command). When this bit is 1, the SPIFI hardware willcontinue to read bytes in the input status field until a test specifiedby the dataLen field is met. The hardware captures each such byte sothat it can be read from the data register, and tests the bit in eachstatus byte selected by dataLen bits 2:0, until a bit is found that isequal to dataLen bit 3. At that time the SPIFI terminates the command byraising CSn. The end-of-command interrupt can be enabled to informsoftware when this occurs. 13:0  dataLen Except when pollRS is 1 asdescribed above, this field controls how many data bytes are in thecommand. 0 indicates that the command does not contain a data field. Allones indicates that the command can include any number of data bytes.

The address register is used as follows. When a command includes anaddress field, the address is written to the address register before theassociated command is written to the command register. For commands thatuse 3-byte and 4-byte address fields, only the number ofleast-significant bits of the address register given by the AMSB fieldof the control register are sent to the device. Any more-significantbits of the address field are sent as zeroes as described in Table 2.The contents of the address register are captured into a workingregister when the command register is written, so that the addressregister can be written again in preparation for a subsequent command.

An intermediate data register is used as follows. Before writing acommand that requires specific intermediate byte values, the value ofthe byte(s) is written to the intermediate data register. In the eventthat more than one intermediate byte is specified in the commandregister, the LS byte written to the intermediate data register is sentfirst. If more intermediate bytes are specified in the command registerthan were provided in the most recent write to this register, zeroes aresent in the subsequent bytes. The contents of the intermediate dataregister are captured into a working register when the command registeris written, so that the intermediate data register can be written againin preparation for the next command or for memory mode. If the CLRID bitin the control register is 1, the intermediate data register is thencleared so that future commands containing intermediate data fields willhave zeroes in them unless the intermediate data register is rewrittenfor each command. For serial flash devices that require dummy (delay)bytes in certain commands, the intermediate data register need not bewritten before issuing those commands.

An address and intermediate register address that is restricted to wordaccesses is used as follows. An address and an intermediate byte valueare written to the register in one write operation for commands that useboth fields with less than 4 bytes of address. When a word is written tothis address, bits 23:0 are written to the address register and bits31:24 are written to bits 7:0 of the intermediate data register. In someimplementations, this register address can be used to set a flashaddress and an “M7-M0” byte for Winbond devices with 16 MBytes/128 Mbitsor less (available from Winbond of San Jose, Calif.).

The data register is used as follows. After initiating a command thatincludes a data output field by writing to the command register, theoutput data is written to the data register. Store Byte instructionsprovide one data byte, Store Halfword instructions provide two bytes,and Store Word instructions provide 4 bytes of output data. Storecommands are delayed if the FIFO 340 is too full to accept the number ofbytes being stored. For Store Halfword and Store Word, the LS byte issent first. After initiating a command that includes a data input fieldby writing to the command register, input data is read from the dataregister. Load Byte instructions deliver one data byte, Load Halfwordinstructions deliver two bytes, and Load Word instructions deliver 4bytes of input data. Load commands are delayed if the FIFO 340 does notcontain the number of bytes being loaded. For Load Halfword and LoadWord commands, the LS byte was received first. In one implementation,for commands that include a dataLen value other than all-ones, no morethan (dataLen) bytes are read from or written to the data register, andthe data register is not used in connection with reading the flashaddress range in the memory map. In DMA transfers in peripheralto-or-from-memory mode, the data register is set as the peripheraladdress.

A memory command register is used as follows. Before accessing the flasharea of the memory map, the SPIFI 300 is set up using the registersdescribed above. A word is then written to the memory command registerto define the command that will be sent to the serial flash to read datafrom the device, when data is read from the flash area. Thereafter, datacan be read from the flash memory area, either directly or using a DMAchannel When the memory command register is written, the SPIFI 300 isset in memory mode. In one implementation, the content of the memorycommand register are identical to that of the command register, exceptas shown in Table 4.

TABLE 4 Memory Command Register Differences from Command Register Bit(s)Name Description 23:21 frameForm 000-011: these commands have no knownuse in this register 110-111: the first time that the bus masteraccesses data from the serial flash memory region after the commandregister was written, these commands send the opcode in bits 31:24 ofthis register, so that 11x operates like 10x. For subsequent commands,caused by timeouts or breaks in ascending address sequence, the opcodeis not sent, so that operation is like when 11x is written to thecommand register. 15:14 DOUT, These bits should be written as 0. pollRS13:0  dataLen Do not write 0 to this field in this register. All onescan be written to this field to indicate that any amount of data can beread in one command (as long as a timeout does not occur). Othernon-zero values will cause the command to be automatically terminatedafter that number of bytes is read, so that a new command will be issuedto the device if the bus master reads more sequential data.

In some implementations, the registers 312 also include a statusregister. The status register is a read-only register that indicates thestate of the SPIFI 300, and includes the fields shown in Table 5.

TABLE 5 Status Register Bit(s) Name Description 0 MCINIT This bit is 1if the memory command register has been written since Reset. 1 CMD Thisbit is 1 while a command is in progress between the SPIFI and the serialdevice. A 1-to-0 transition on this bit causes a SPIFI interrupt requestif the INTEN bit in the control register is 1. 2 MCMD This bit iscleared by Reset and by a write to the command register. It is set whena command is initiated by a read from the memory area with MCINIT 1. 3CMDI This bit is cleared at the start of a command, and set if and whenthe command begins reading from the serial flash. 12:8 FIFObytes Thisfield indicates the number of bytes currently in the FIFO. 0 means theFIFO is empty.

According to an example embodiment of the present invention, the flashinterface (e.g., SPIFI 300) is configured to connect to an ABB(high-performance bus) and operates in accordance with signals asrepresented in Table 6, as described further below.

TABLE 6 SPIFI AHB Signals Signal Direction Description HCLK In AHBclock. Most of the SPIFI logic is clocked by this signal. HRESETn InLow-active master reset. HSELmem In A high on this signal indicates anaccess to the serial flash memory region. HSELreg In A high on thissignal indicates an access to the SPIFI registers. HADDR31:0 In AHBaddress. When HSELreg is high, the SPIFI need only decode HADDR2 andenough more significant address lines to differentiate among the variousSPIFI registers. That is, the SPIFI registers are aliased throughout theaddress region allocated for them. When HSELmem is high, the SPIFIcaptures the state of HADDR0 up through the bit number specified by theAMSB field in the control register, and should pass that address to theserial flash device when necessary. After each access to the serialflash memory region is completed, the SPIFI hardware should continue toaccess (higher-addressed) serial flash data as long as the FIFO is notfull and the bus master does not request other data. HRDATA31:0 Out AHBread data. These are byte-laned, such that data for byte addresseshaving the following values of HADDR1:0 should be returned on the linesindicated: 00 HRDATA7:0 01 HRDATA15:8 10 HRDATA23:16 11 HRDATA31:24HWDATA31:0 In AHB write data. These are byte-laned: data for byteaddresses should be captured from these lines in the same fashion shownfor HRDATA above. HTRANS1 In Indicates the type of bus cycle. The SPIFIshould do nothing in clocks when HTRANS1 is 0 and/or both HSEL signalsare low. It should respond when one of the HSEL signals is high andHTRANS1 is 1. HWRITE In Indicates the direction of transfer when HTRANS1is 1 (H = write). HSIZE2:0 In Indicates the width of the requested datatransfer when HTRANS1 is 1: 000 Byte 001 Halfword 010 Word The SPIFIresponds with an ERROR on the HRESP lines if these lines carry a valuegreater than 010. HRESP1:0 Out Indicate whether a requested transfercould be completed: 00 OKAY transfer not yet completed, or successfullycompleted 01 ERROR results in an abort on the bus master HREADY Out Lowindicates that the requested transfer is not yet complete. Driven low inthe following situations: When the bus master reads an address in theserial flash region, and the data for that access is not completelypresent in the FIFO. When the bus master reads the data register, andthe width of data requested is not yet in the FIFO. When the bus masterwrites to the data register, and the FIFO doesn't have room for the dataprovided.

According to another example embodiment, a SPIFI (e.g., 300) connects tothe signals shown in Table 7, relative to the serial flash and otherportions of the device containing the flash interface.

TABLE 7 SPIFI I/O Signals Signal Direction Description SCKI In Serialclock. This signal clocks the serial shift register, and is divided by 2to produce SCKO during active commands. SCKO Out Clock to the SCK padcell or the in-package serial device CSn Out Chip Select to the CSn padcell or the in-package serial device I0 In Input from the IO0 pad cellor the in-package serial device O0/MOSI Out Output to the IO0/MOSI padcell or the in-package serial device OEN320 Out Controls the directionof the IO0/MOSI and IO3:2 pad cells I1/MISO In Input from the IO1/MISOpad cell or the in-package serial device O3:1 Out Outputs to the IO3:1pad cells or the in-package serial device OEN1 Out Controls thedirection of the IO1/MISO pad cell I3:2 In Inputs from the IO3:2 padcells or the in-package serial device INTRQ Out Interrupt request to theinterrupt controller. Set at the end of a command if the INTEN bit is 1.Cleared by Reset, by reading the Status register, and by writing theCommand register. GRANTn In Ground this input to disable multi-masteroperation. Otherwise, connect it to a low-active grant signal from anexternal arbiter that will be high by the trailing edge of Reset. TheSPIFI captures the state of this input at the end of Reset. It it's lowat the end of Reset, the SPIFI drives OEN320 high after an inputcommand, and OEN1 high after a quad input command, 2 SCKI periods (1SCKO period) after driving CSn high. If this input was high at the endof Reset, the SPIFI drives OEN320 and OEN1 low (if necessary) 1 SCKIperiod after driving CSn high between frames, and after it asserts CSnlow it waits for this input to be low before asserting OEN320 high (andOEN1 high in Quad mode). The multi-master mode increases the timerequired to send each command by 2 SCKI periods (1 SCKO period) forsynchronization of GRANTn. DMACBREQ Out DMA Request DMACCLR In DMARequest Clear signal

FIG. 5 shows an example startup sequence through a first memory modeaccess for an arrangement 500, according to another example embodiment.A master controller (microcontroller unit MCU) 502 controls a slaveflash memory 504, using a sequence of steps 510-522, one or more ofwhich may be implemented in accordance with the description above. Forexample, after reset, the master sends an identification read command512 to the slave, which responds by sending its identification data 514,such as manufacturer ID and device ID. This information can be used inconfiguring subsequent communications and operations with the slave 504,such as by using the identification to determine a command set thatworks with the flash memory 504. Writes are performed in steps 510(timeout), 512 (command), 516 (control register) and 518 (memory controlregister), and reads are done in steps 514 and 522.

FIGS. 6A and 6B show a system 600 including an MCU 610, a flashinterface circuit (SPIFI) 620 and a flash memory circuit 630 operatingin memory mode without and with timeout, respectively. Addresses arerepresented in hexadecimal (base 16) notation. The SPIFI 620 and flashmemory circuit 630 may operate in accordance with one or more exampleembodiments as discussed herein, with various control and data passed asshown. For example, when the MCU 610 requests a word at 424 from theSPIFI 620, the SPIFI sends a wait response to the MCU while data isretrieved, and sends a memory command including address 424 to the flashmemory circuit, which responds by sending data to the SPIFI to meet therequest and thereafter to fill the FIFO with sequential addresses.

When the MCU 610 then requests a word at 42C, the SPIFI 620 discardswords 428-42B, returns the requested word to the MCU from the FIFO, andreads additional sequential addresses from the serial flash 630 to fillthe FIFO.

When the MCU 610 then requests a byte at 600, the SPIFI 620 sends a waitresponse to the MCU, clears the FIFO because byte 600 is not present inthe FIFO, drives CS high for one clock to terminate the previouscommand, and then sends a memory command including address 600 to theflash memory circuit 630. In response, the flash memory circuit 630sends two clocks to obtain the data for the byte at 600. The SPIFIcontinues to produce additional clock pulses until the FIFO buffer isfilled from subsequent addresses.

FIG. 6B shows similar operation, with a timeout occurring betweenproviding words 424 and 428, and the provision of word 428 from the FIFObuffer to the MCU 610 without delay, but with activity resumedthereafter to re-fill the FIFO.

FIG. 7 shows a FIFO block diagram 700 for implementation with a FIFO andflash memory circuit, according to another example embodiment of thepresent invention. The FIFO includes sixteen byte registers numberedF15-F0 and stands between a shift register that transfers data to orfrom serial flash and HWDATA and HRDATA buses that transfer data from orto a bus master. In some implementations, the bus interface and most ofthe FIFO are clocked by the bus clock, the shift register(s) is (are)clocked by SCKI, and synchronization logic is either built into the FIFOor placed between the FIFO and the shift register(s).

When there is data in the FIFO, F0 contains the byte that is next to bedelivered to the shift register or that will be placed on HRDATA7:0 inthe next read operation by the bus master. When the FIFO is full, F15contains the last byte stored from the shift register, or the byte fromthe most significant byte in the last write to the data register. Theoutputs of F0 are connected to the parallel-load inputs of the shiftregister. During a read from the serial flash address region or a readfrom the data register, the outputs of F0 are driven onto HRDATA7:0. Theoutput of F1 is driven onto HRDATA15:8 for a halfword or word read, andoptionally for a byte read. The outputs of F3-F2 are driven ontoHRDATA31:16 for a word read and optionally for a halfword or byte read.

The parallel outputs of the shift register and HWDATA are connected to awrite data router having four 8-bit outputs called Shifted Byte Lanes,or SHBL0[7:0] through SHBL3[7:0]. SHBL0 is the “write input” for byteregisters F0, F4, F8, and F12. SHBL1 is the write input for F1, F5, F9,and F13. SHBL2 is the write input for F2, F6, F8, and F14. SHBL3 is thewrite input for F3, F7, F11, and F15. SHBL3 goes directly to the Dinputs of F15. The D inputs of F14-F0 are connected to the outputs ofmultiplexers, each of which selects between its write input and the Qoutputs of one (F14), two (F13-F12) or three (F11-F0) “shift inputs”from higher-numbered byte registers. Each byte register also has anenable input that controls whether data is clocked from its D inputs toits Q outputs on each rising edge of HCLK.

The DOUT bit in the command register controls the direction of data flowbetween the bus master and the shift register during commands written tothe command register. If DOUT is 1, data is written to the FIFO usingthe address of the data register, and the data is transferred throughthe FIFO to F0 and then to the shift register and the serial flash. IfDOUT is 0 (and for accesses to the serial flash address region) data iswritten into the FIFO from the shift register, and the data travelsthrough the FIFO to F0 to F3, from which it is read by the bus master.

The operation of the FIFO is also controlled by a “bytes in FIFO”register. This 5-bit register can contain 0 indicating that the FIFO isempty, through 16 indicating that the FIFO is full. The number of“retained bytes” is “bytes in FIFO” less the number of bytes that willbe removed from the FIFO at the end of the clock. The number of bytesremoved from the FIFO is 1 in a clock at the end of which write datawill be loaded into the shift register, 1 in a clock in which a byte ofdata is read (with HREADY high) by the bus master, 2 in a clock in whicha halfword of data is read with HREADY high, 4 in a clock in which aword of data is read with HREADY high, and 0 otherwise.

During each clock, “next bytes in FIFO” is the “retained bytes” valueplus the number of bytes written into the FIFO in that clock. Thisresult is loaded into the “bytes in FIFO” register at each clock risingedge. The number of bytes written into the FIFO is 1 in a clock at theend of which data is written into the FIFO from the shift register, 1 ina clock in which a byte of data is written into the FIFO by the busmaster with HREADY high, 2 in a clock in which a halfword of data iswritten by the bus master with HREADY high, 4 in a clock in which a wordof data is written by the bus master with HREADY high, and 0 otherwise.In a clock in which the number of bytes removed from and/or written intothe FIFO is (are) non-zero, and the “next bytes in FIFO” value isgreater than zero, the enable lines of byte registers F0 through F[“nextbytes in FIFO”−1] are asserted so that data is shifted and/or loaded inthe FIFO.

The control inputs of the input multiplexers for each byte register “i”are driven to one of four states “write,” “byte shift,” “halfwordshift,” and “word shift.” The “shift” values are driven only in clocksin which bytes are being removed from the FIFO, and then only to byteregisters for which i<“retained bytes”. “Word shift” is driven when thebus master is reading a word with HREADY high, “halfword shift” isdriven when the bus master is reading a halfword with HREADY high, and“byte shift” is driven when the bus master is reading a byte with HREADYhigh or a byte is being transferred from F0 to the shift register. Inputmultiplexer controls are set to “write” in all other cases and clocks,although writing occurs only for registers whose Enable inputs areasserted as described above.

The input multiplexers for F0-F11 select among their SHBL write input,the output of F[i+1] for a byte shift, the output of F[i+2] for ahalfword shift, and F[i+4] for a word shift. The input multiplexers forF12 and F13 select among their SHBL write input, the output of F[i+1]for a byte shift, and the output of F[i+2] for a halfword shift. Theinput multiplexer for F14 selects between the SHBL2 write input and theoutput of F15 for a byte shift. F15 has no input multiplexer.

The write data router routes the byte from the shift register to allfour SHBL buses when DOUT is 0 or during accesses to the serial flashaddress region. When a command initiated by writing DOUT=1 to thecommand register is in progress, the write data router routes the byteson HWDATA31:0 onto the SHBL lines under control of the two leastsignificant bits of the “retained bytes” value as shown in Table 8.

TABLE 8 Write Data Routing for direct command with DOUT = 1 HWDATA byteplaced on “retained bytes”[1:0] SHBL3 SHBL2 SHBL1 SHBL0 00 31:24 23:1515:8  7:0 01 23:15 15:8  7:0 31:24 10 15:8  7:0 31:24 23:15 11 7:0 31:2423:15 15:8 

According to another example embodiment of the present invention, aSPIFI device is configured to operate in a peripheral mode DMAoperation. The HREADY line is driven and wait states are inserted whennecessary during read and write operations by the bus master, tomaintain synchronization between bus master accesses and serial datatransfer with a serial flash memory circuit. This mechanism facilitatesload and store accesses and “memory to memory” transfers by a DMAchannel.

According to another embodiment of the present invention, a SPIFI isconfigured to operate in a mode that supports DMA transfers in which theSPIFI acts as a peripheral and drives a request signal to a DMA channelto control data transfer. This mode is different from “memory to memory”operation, and may be useful in systems in which the code and/or data isdynamically fetched from the serial flash into RAM on as “as needed”basis. Clock cycles are thus not lost to wait states, and the overalloperation of the processor and DMA channels is more efficient. In oneimplementation, the DMA peripheral mode is operated as appropriate forthe ARM PL080 DMA Controller (DMAC) or related controller available fromARM of San Jose, Calif. The DMAC is programmed/configured to presentword read operations at the fixed address of the data register, to havea source burst size of 2 words, and a transfer size of an even number ofwords. The SPIFI drives a DMACBREQ signal to the DMAC, and receives aDMACCLR signal from it. To use the DMA peripheral mode, the commandregister is written to start the command, and a DMA channel isprogrammed as described above, to read data from the data register andwrite it into RAM. The SPIFI asserts DMACBREQ when: a command is inprogress (CMD is 1), MEMMODE and DOUT are both 0, there are 8 or morebytes in the FIFO, and DMACCLR is negated (low).

The various embodiments as discussed herein may be implemented using avariety of structures and related operations and functions. Forinstance, the bus master may communicate with the flash interface viaseparate buses to access the registers and the mapped flash region, orvia a single bus for access to both sets of addresses. As anotherexample, while most of the descriptions herein may involve software orfirmware that plays a role in adapting to different flash memorydevices, various embodiments are directed to implementations in whichthe hardware includes all necessary resources for such adaptation,without necessarily requiring any involvement of software and/orfirmware. Also, various descriptions herein show include hardware havinga number of interacting state machines, but aspects of these and otherembodiments may include implementations in which the hardware isorganized into a different set and/or number of state machines,including a single state machine, as well as random-logicimplementations that may not be clearly mapped to any number offinite-state machines. While various embodiments can be realized viahardware description language that is computer-synthesized to a libraryof standard modules, aspects of the invention should also be understoodto cover other implementations including, but not limited to,field-programmable or masked gate arrays, seas of gates, opticalcircuits, board designs composed of standard circuits, microcodeimplementations, and software- and firmware-dominated implementations inwhich most or all of the functions described as being implemented byhardware herein are instead accomplished by software or firmware runningon a general- or special-purpose processor. These embodiments may alsobe used in combination, for example certain functions can be implementedusing programmable logic that generates an output that is provided as aninput to a processor.

Based upon the above discussion and illustrations, those skilled in theart will readily recognize that various modifications and changes may bemade to the present invention without strictly following the exemplaryembodiments and applications illustrated and described herein. Suchmodifications and changes may include, for example, using interfacecharacteristics incorporating one or more of a variety of approaches forSPI-based memory operation and communications. One such example involvesflash-type memory devices and operational characteristics of devicessuch as those available from Winbond of San Jose, Calif., and fromSilicon Storage Technology of Sunnyvale, Calif. These and othermodifications do not depart from the true spirit and scope of thepresent invention, including that set forth in the following claims.

1. A peripheral interface circuit for interfacing between a computerprocessor and a flash memory circuit communicatively connected to theinterface by a peripheral interface bus, for storing and providingaccess to data that is stored on the flash memory circuit and mapped asa portion of main processor memory, the interface circuit comprising: afirst-in, first-out (FIFO) buffer coupled to receive data from and storedata for the flash memory circuit and to provide read access to thestored data; and an interface controller configured to communicate withthe flash memory circuit via the peripheral interface bus, initializethe flash memory circuit for providing data to the FIFO buffer as aportion of the main memory, in response to a request for data startingat an address in a portion of main memory and having at least a portionthereof stored in the FIFO buffer, control the FIFO buffer to provideaccess to the stored data and control the flash memory circuit toprovide additional data from subsequent addresses in the flash memorycircuit to the FIFO buffer, and in response to a request for datastarting at an address in portion of main memory and not stored in theFIFO buffer, control the flash memory circuit to provide the requesteddata and additional data from subsequent addresses in the flash memorycircuit to the FIFO buffer.
 2. The interface circuit of claim 1, whereinthe interface controller is configured to communicate with differenttypes of said flash memory circuits respectively operating on disparatecommands, by communicating with the flash memory circuit to retrieveidentification information from the flash memory circuit, and inaccordance with the retrieved identification information, select and usea particular command set for initializing, communicating with andcontrolling the flash memory circuit.
 3. The interface circuit of claim1, further including a register having control information including anopcode field containing an opcode, a frame format field that indicateswhether the opcode is to be sent and how many address bytes are to besent, a parallel/serial field that indicates which of the fields in thecommand are sent or received in serial SPI protocol, and which of thefields in the command are sent or received in 4-bit parallel (quad)protocol, an intermediate length field indicating how many intermediatedata bytes are to be sent between the opcode and/or an address, and thedata, a data out bit controlling whether data is sent to or receivedfrom the flash memory circuit, a poll read status bit controllingwhether the flash interface should read data bytes continuously from theflash memory circuit until a byte is received that meets a conditionselected by a data content field, and said data content field forcontrolling different operational aspects based upon the poll readstatus bit, including for a poll read status bit of 0, indicating thenumber of bytes of data to be sent or received in a command, with anall-ones value indicating an unlimited number, and for a poll readstatus bit of 1, containing three bits selecting which bit of each byteread from the flash memory circuit is to be tested for commandcompletion, and a fourth bit indicating the bit state that indicatescompletion of the command; and wherein the interface controller isconfigured to determine the format and operation of a command to be sentand performed with the flash memory circuit, based upon the data in theregister.
 4. The interface circuit of claim 1, wherein the interfacecontroller is configured to operate the flash memory circuit in an erasemode to erase data on the flash memory circuit, operate the flash memoryin a programming mode to store data in the flash memory, and receive andstore the data in the flash memory circuit mapped as a portion of mainmemory in a read-only mode.
 5. The interface circuit of claim 1, whereinthe interface controller is configured to interface with the computerprocessor to receive the requests for data and to control the FIFObuffer for providing the requested data for use by the processor.
 6. Theinterface circuit of claim 1, wherein the interface controller isconfigured to interface with a peripheral device via the peripheralinterface bus to receive requests for data from a bus master and tocontrol the peripheral device and FIFO buffer to provide the requesteddata to the bus master.
 7. The interface circuit of claim 1, wherein theinterface controller is configured to identify a type of the flashmemory circuit and to communicate with the flash memory circuit using acommand set and communications protocol for the identified type.
 8. Aflash memory-interface system for storing and providing access to datamapped as a portion of main memory of a computer, the system comprising:a peripheral interface bus; a flash memory circuit configured to connectto and communicate via the peripheral interface bus and to provide thedata in the flash memory mapped as a portion of main memory; a first in,first out FIFO buffer coupled to receive and store data from the flashmemory circuit and to provide access to the stored data; and aninterface circuit configured to communicate with the flash memorycircuit via the peripheral interface bus, initialize the flash memorycircuit for providing data to the FIFO buffer as a portion of the mainmemory, in response to a request for data starting at an address in saidportion of main memory and having at least a portion thereof stored inthe FIFO buffer, control the FIFO buffer to provide access to the storeddata and control the flash memory circuit to provide additional datafrom subsequent addresses in the flash memory circuit to the FIFObuffer, and in response to a request for data starting at an address ina portion of main memory and not stored in the FIFO buffer, control theflash memory circuit and FIFO to provide the requested data to the FIFObuffer and, thereafter, to provide additional data from subsequentaddresses in the flash memory circuit to the FIFO buffer.
 9. The systemof claim 8, wherein the interface circuit is configured to communicatewith the flash memory circuit to retrieve identification informationfrom the flash memory circuit, and in response to the retrievedidentification information, set and use a command set for communicatingwith, initializing, and controlling the flash memory circuit.
 10. Thesystem of claim 8, wherein the interface circuit is configured to, inresponse to the request for the data starting at an address in a portionof main memory and having at least a portion thereof stored in the FIFObuffer, control the flash memory and the FIFO buffer to provide accessto any previously unread portion from subsequent addresses in the flashmemory circuit to the FIFO buffer, followed by reading and storing datafrom subsequent addresses in the flash memory circuit in the FIFObuffer.
 11. The system of claim 8, wherein the interface circuit isconfigured to, in response to the request for data starting at anaddress in a portion of main memory that is not stored in the FIFObuffer, provide access to the requested data, and thereafter, controlthe flash memory and the FIFO buffer to read and store data fromsubsequent addresses in the flash memory circuit in the FIFO buffer. 12.The system of claim 8, wherein the interface circuit is configured tooperate the flash memory circuit in a programming mode to store data onthe flash memory circuit.
 13. The system of claim 8, wherein theinterface circuit is configured to send a programming command over theperipheral interface bus to the flash memory circuit to operate theflash memory circuit in a programming mode, and after said programmingcommand is completed, send a status-polling command to the flash memoryto detect completion of the resulting flash programming operation. 14.The system of claim 8, wherein the interface circuit is configured tooperate the flash memory circuit in an erase mode to erase data on theflash memory and to operate the flash memory circuit in a programmingmode to store data on the flash memory circuit, after the data on theflash memory is erased.
 15. The system of claim 8, wherein the interfacecircuit is configured to operate the flash memory circuit in a read-onlymemory mode to provide read-only access to data stored in the flashmemory circuit as a part of the main memory space of an associatedprocessor.
 16. The system of claim 8, wherein the interface circuit isconfigured to provide a clock signal to the flash memory circuit forreading data out from the flash memory circuit to the FIFO buffer, andto cease switching said clock in response to the FIFO buffer beingfilled.
 17. The system of claim 8, wherein the interface circuit isconfigured to provide a chip select signal to the flash memory circuitfor reading data out from the flash memory circuit to the FIFO buffer,and to de-assert the chip select signal when the controlling bus masterhas not accessed any data from the flash memory for a programmed timeperiod.
 18. A method for interfacing between a computer processor and aflash memory circuit communicatively connected to an interface circuitby a peripheral interface bus, for storing and providing access to datathat is stored on the flash memory circuit and mapped as a portion ofmain processor memory, the method comprising: initializing the flashmemory circuit, via the peripheral interface bus, for providing data toa first-in, first-out (FIFO) buffer coupled to receive and store datafrom the flash memory circuit and to provide read access to the storeddata as a portion of the main memory; in response to a request for datamapped in said portion of main memory and having at least a portionthereof stored in the FIFO buffer, controlling the FIFO buffer toprovide access to the stored data and controlling the flash memorycircuit via the peripheral interface bus to provide additional data fromsequential addresses in the flash memory circuit to the FIFO buffer; andin response to a request for the data mapped as a portion of main memorythat is not stored in the FIFO buffer, controlling the flash memorycircuit via the peripheral interface bus to provide the requested datato the FIFO buffer.
 19. The method of claim 18, further includingcommunicating with the flash memory circuit via the peripheral interfacebus to retrieve identification information from the flash memorycircuit, and in response to the retrieved identification information,set and use a command set for initializing, communicating with andcontrolling the flash memory circuit via the peripheral interface bus.